Frequency modulation signal receiver



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FREQUENCY MODULATION SIGNAL RECEIVER Filed Oct. 26, 1940 Bawag/wx .l EB .Q Y y I jjgfomey AV-Wcmf IN ,329/139 Snvenfor n' I rmf-.iraffmi-m... .i` f www. .........,..T......

Patented Nov. 25, 1941 v' 2,264,151 y i A FREQUENCY MonoLA'rIoN SIGNAL f RECEIVER John D. Reid, Philadelphia, Pa., assigner fp nadia Corporation of America, a corporation of Dela- Ware Application opioberzs, 1940, serial No. 362,912

scxaims. (Grasa-2o) This invention relates to frequency modulation signal receivers, and has for its primary object to provide an improved frequency modulation signal conveying system comprising a series of tuned 4signal selecting circuits terminating in a fre. quency discriminator network and detector," the overall sensitivity characteristic Qf` w hich has substantially zero slope in-the frequency range of an interfering signal """WM" In frequency modulation signal receiving systems, the `overall selectivity characteristic of 'a series of signal selecting circuits, preceding a discriminator network and second detector, provides a certain band width of response, for example, a band 200 kc. wide, and a selectively characteristic having substantially a fiat top and sloping sides on eitherl side of the pass band. The signal output or audio frequency signal derived from a detector comprising a balanced rectifier and frequency discriminator network in connection with such circuits, in response toreceived frequency modulated signals, is determined by the slope of the selectivity characteristic, and the ysteeper the sides ofsuch characteristic, the greater the signal'output with a lgivenfrequency swing.

Therefore, in the example above referred to, signals in the region of adjacent channel signal frequencies, which may have center frequencies substantially 200 kc. on either side of the center frequency of a. selectedA signal, may fall within the selectivity characteristic and, while intercepting the slope on either side of the pass band at relatively low amplitude levels, may cause an appreciable output and interference in the discriminator network and detector. In other'words, a

signal that isrelatively low in amplitude with respect to the received signal may fall within the pass band of the signal selecting circuits and intercept the selectivity characteristic at a point of relatively sharp slope sufficient to provide a high output in the discriminator circuit and heavy interference.

It has been found that this interference may substantiallybe prevented by imparting to the selectivity characteristic of one or moreof the 'i "`\\-sig nal selecting circuits, a reversal of. slope such that the overall selectivity characteristic at the discriminator network output circuit and at the second detector has substantiallya zero slope or horizontal portion on either side of a desired pass band and in the region of an adjacent channel or interference signal frequency. In a preferred embodiment of the invention, in accordance with this system, thesignal output to the audio frequency or second detector is made sub'- .stantially zero in response to adjacent channel and other signals in frequency ranges on either side of the desired pass band. f

The invention is particularly adapted for use in connection with the intermediate frequency amplifier of. a superheterodyne receiver and other -cascaded signal conveying circuits tuned to a preselected signal band, such as an intermediate frequency band about a predetermined center or mid-frequency. l

In,` carrying the inventionv into effect inv its present preferred form, in connection with an Aintermediate frequency amplifier, one or more of the successive tuned circuits, such as the primary or secondary circuit or both circuits of a tuned intermediate frequency coupling transformer, are provided with tunable rejection circuits effective tolprovide a reverse slope in one or both sides of the selectivity characteristic of the tuned circuit or circuits to whichl they are connected, the reversal of slope being to' av degree sufficient to effect a flattening oir-*reduction to subtsantially zero of thef slope of the overall selectivity characteristic at the discrirninatorr network, thereby to provide substantially zero response to aV predetermined interfering signalv or adjacent channel signals. y

The invention will, however, be further understood from-the following description when considered in connection with the accompanying drawing, and its scope is-pointed out in the appended claims.

' In'. the drawing,

Figure 1 is a schematic circuit diagramv of 'a frequency modulation signal receiving system embodying the invention, and

Figures 2, 3` and 4 are curves showing selectivity characteristics at certain points in the receiving system of Fig. 1.

Referring to Fig. l, an intermediate frequency amplifier 5 comprising tuned circuits 6 and lis connected at its output end with a frequency discriminator network B provided with a pair of balanced rectiflers 9 and Hl, and at its input end to suitableV tuning and signal receiving means including detector' and oscillator elements as indicatedatll. The intermediate frequency amplifier comprises a rst stage amplifier tube I2 coupled through the tuned circuit 6 to aseccnd stage amplifier i3. The circuits 6 may include thetuned primary and secondary windings of an interstage intermediate frequency coupling transformer I4, the selectivity characteristic of which is indicated by 2- the curve I5 in Fig. 2. It will be noted that the selectivity'characteristic has substantially a flat top providing a predetermined pass band, and that the sides of the characteristic curve are sloped at points of intersection I6 and I1 with interference signal frequencies Fl and F2 which may be the center frequencies of adjacent signal channelsr The center frequency of the desired signal channel is indicated at F6.

A third intermediate frequency amplifier stage comprises a tube 20 which is coupled to the second stage i3 through a similar tunedinterstage coupling transformer 2| as is provided att I4; This is included in the circuit T which also includes a rejection circuit 22' on'tlierprimary'side and a similar rejection circuit'23ton the' secondary side, the said circuits being tuned to; modify the selectivity characteristicl lLon-each'sidefinj accordance with the selectivity characteristic 25 of Fig. 3, to provide reverse slopes 26 and 2'!- therein; with the frequencies-Fl and F21 falling substantially in the center. of the-.reverse slopes asshown. e

A suidcient number of rejectionA circuits are provided in the signal channel preceding the discriminator network 8 to provide a reverse slope on one or both sides of the selectivity curve of one or more of the series of tuned circuits in order to impart a horizontal or substantially Zero slope; in` the overall selectivity characteristic of the discriminator network 8.

The network 8 may be of any suitable type responsive to variation in frequency about a predeterminedn center or meanfrequency, and in the present example comprises a circuit 3U tuned above and a circuit 3l tuned below the center frequency and coupled to the output circuit 32 of the last intermediate frequency amplifier stage 2li. The rectiers 9 are connected to the circuitsY 30 and 3l in balanced relation thereto and to a balanced output circuit comprising output resistors 33 and 34 connected in series betweenthe rectiers 9 and l0. The output circuits/SS-and 3E may be connectedv to any suitable utilization means such as an audio frequency amplifier` 3'? followed by a loudspeaker device 38 as shown.

Referring to Fig; 4 along with the preceding figures, the selectivity characteristics-4l! and 4l are those taken at the circuits 3i) and 3| respectively showing the voltage output el and e2 for said circuits with variation in frequency above and below the center frequency F.V Y

It will be noted that the effect of ther-rejection circuits in the preceding signal conveying circuits issuch that a portion 42 and a portion N3 in the slopeof the overall characteristic is substantially flat or horizontal in vpredetermined-frequency zones or in the region of undesired signal frequencies FI and F2. Thus the interference from adjacent channel or other predetern'lined` interfering signal source is substantially eliminated by this system. It has been found thatattenuation of a relatively high order may be 'obtained in this manner in a frequency modulation signal receiving systemV such that practically noA adjacent channel interference is experienced in operation.

By varying the tuning, coupling and damping of the tuned circuits 22 and 23, the position of the zero slope portions42 and 43 of the overall characteristics may be shifted.

I'claim as my invention:

1'. In a frequency modulation signal receiving system, the combination of a signal amplifier including a plurality of cascade-connected signalselecting circuits, means for coupling and tuning each of said circuits to provide a selectivity characteristic having a slope on either side of a predetermined passband, means for imparting to the selectivity characteristic of at least one of said circuits a reversal of slope of such degree that the overall selectivity characteristic of said circuits provides substantially zero slope in a frequency zone on at least one side of said passband,

Vand a tuned frequency discriminator network coupled to the amplifier to utilize said selectivity characteristicto provide substantially zero output in said frequency zone.

2.v In a frequency modulation signal receiving of said circuits having a selectivity characteristic providing a slope on either side of a predetermined-passband, a frequency discriminator network connected with said amplifier, and means forimparting to the selectivity characteristic of at least one of said circuits a reversal of slope of such degree-that the overall selectivity characteristic'of said circuits at the output of said discriminator network provides substantially zero slope in a frequency zone on either'side of said passband.

3; Ina frequency modulation signal conveying system comprising a. plurality of: tuned cascadeconnected signal selecting circuits, the method of rejecting undesired frequency modulated signals which comprises causing the overall selectivity characteristic of said circuits to have substantially zero slope in the frequency range of an interfering signal. t v

4.*In a frequency modulationtsignal conveying system comprising a plurality of tunedy signal selecting circuits, the method of rejecting undesired frequency modulated signals which comprises imparting to at least one of'said circuits a band pass selectivity characteristic, modifying the: frequency response of said circuittoimpart to saidselectivity charactistica reverse slope substantially centered in the frequency range of an interfering signal, and extending the degree to which said frequency response is modi'ed to providean overall. selectivity characteristic for said circuits' havingv substantially zero-slope in said last named frequency range.

5. In a frequency modulationsignal conveying system, the combination with a plurality of successive band pass circuits tuned to a common center'frequency, of an absorption circuit coupled to'one of said circuits and tuned to signals in a frequency range about a predetermined adjacent channel frequency for imparting to the selectivity characteristic of said last named circuit a reverse slope on at least one side thereof', means for controlling said absorption circuit to impart substantially a zero slope to the overallselectivity characteristic'of said system insaid frequency range, and a discriminator network coupledA to the output end of said circuits and tuned for response to signals within the pass band thereof to effect substantially zeroresponse and signal output in said adjacent channel frequency range.

6. In a superheterodyne frequency modulation signal receiving system, the combination of an intermediate frequency amplifier having a plurality of cascade connected tuned amplifier circuits responsive to Ysignals in a predetermined frequency band having a predetermined center frequency, a .frequency discriminator network coupled to said amplifier, a pair of rectiers connected to saiddisoriminator network providing rif.

a balanced detector, and means for imparting to the selectivity characteristic of one of said amplier circuits a reversal of slope effective to provide substantially zero slope in the overall frequency characteristic at the output of the discn'minator networkin the region of aninterfer ing signal frequency.

7. In a superheterodyne frequency modulation signal receiving system, the combination of an intermediate frequency amplier having a plurality of cascade connected tuned amplifier stages responsive to signals in a predetermined fre-A quency band having a predetermined center irequency. a frequency dscriminator network coupled to said amplier, a pair of rectiers connected to said discriminator network providing a balanced detector, a balanced output circuit connected with said rectiers, and a pair of tuned absorption circuits coupled to certain of said iirst named circuits for imparting to the selectivity ,characteristic of at least one of said amplifier a selectivity characteristic providing a slope on' 20 passband.

oneside of a predetermined passband, a frequency discriminatorv network connected with said amplier, and means for imparting to the selectivity characteristic of said circuitl a reversal of slope of such degree that the overall selectivity characteristic of said circuit at the output of said discriminator network provides substantially zero slope n a frequency zone on said one side of the JOHN D. REID. 

